Spinach (Spinacia oleracea L.) is a flowering vegetable plant belonging to the family Amaranthaceae. It is native to Southwestern and Central Asia, but today spinach is cultivated worldwide, mainly in temperate regions. Spinach plants are cultivated for their highly nutritious leaves. The leaves are extremely rich in antioxidants, a good source of vitamins such as A, B6, C, E, K and folate, and minerals such as calcium, iron, magnesium and potassium.
The edible part of the spinach plant is a compact rosette shape of leaves attached to a short stem. The leaves are produced during the first stage of the plant's life cycle, the vegetative rosette stage. The second stage is the flowering stage, or the bolting stage. When bolting occurs, there is growth of an elongated stalk with flowers growing from within the main stem of the plant. Once the plant has reached the bolting stage, it is no longer possible to harvest marketable leaves. The plant will allocate its resources to flowering instead of leaf production, which will ultimately cause the leaves to wither. Fast bolting is thus a very undesired trait in the production of leafy vegetables such as spinach. Therefore, slower bolting is very much preferred by growers in order to optimize yield.
There are three basic types of spinach, namely savoy, semi-savoy, and smooth. The savoy type spinach has dark green, crinkly and curly leaves. The semi-savoy type spinach is a hybrid variety with slightly crinkled leaves. Smooth type spinach has broad, smooth leaves.
The leaves of a spinach plant are usually sold loose, bunched, in pre-packaged bags, canned or frozen. Each of these products requires its own type of leaves. The smooth and some of the semi-savoy varieties are predominantly used in processed products such as canned or frozen spinach, while the semi-savoy and savoy types are especially used for the fresh market. Smooth leaf varieties are better suited for processing purposes than semi-savoy and savoy varieties, because they are easier to clean and prepare for canning and freezing. Savoy leaf varieties, on the other hand, are preferred in the fresh market because the leaves pack looser than the smooth leaf varieties and are less likely to wilt or turn yellow.
The growth habit of spinach varieties can be classified in three different categories, flat or prostate, semi-erect, and upright or erect. For mechanical harvest an upright or erect habit is preferred in order to reduce soil contamination. Especially for varieties with savoy type leaves this is an important character, because the soil is difficult to remove from curly leaves. Sometimes even plant growth regulators are applied before harvest to cause a more upright leaf growth and reduce the risk of soil contamination. More upright varieties are thus desired by growers to reduce contamination and costs for spraying plant regulators.
Downy mildew is probably the most widespread and potentially destructive global disease of spinach. The causal agent of downy mildew on various Amaranthaceae, including spinach, is regarded as a single species, Peronospora farinosa. In particular, Peronospora farinosa f. sp. spinaciae infects spinach. Initial symptoms of downy mildew consist of dull to bright yellow necrotic lesions that appear on the leaves of infected spinach plants. With time the lesions can enlarge and become tan and dry. The infection can spread very rapidly, and it can occur both in glasshouse cultivation and in soil cultivation, resulting in widespread crop damage. In addition to the loss of quality of the leaves due to the lesions, downy mildew can also cause breakdown and rot of the infected leaves if they packaged in bags and cartons.
The optimal temperature for formation and germination of P. farinosa f. sp. spinaciae spores is 9 to 12° C., and it is facilitated by a high relative humidity. When spores are deposited on a humid leaf surface they can readily germinate and infect the leaf. Fungal growth is optimal between 8 and 20° C. and a relative humidity of ≧80%, and within 6 and 13 days after infection mycelium growth can be observed. Oospores of P. farinosa can survive in the soil for up to 3 years, or as mycelium in seeds or living plants. Although some fungicide treatments may be effective, they are costly and cause ecological pollution.
In recent years various resistance genes have been identified that provide spinach plants with a resistance against downy mildew. However, it has been observed that previously resistant spinach cultivars can again become susceptible to the fungus. Investigations revealed that the cultivars themselves had not changed, and that the loss of downy mildew resistance must therefore be due to P. farinosa overcoming the resistance in these spinach cultivars. The downy mildew races (also called physios, isolates, or strains) that were able to infect resistant spinach cultivars were collected in a differential reference set, which can be used to test spinach cultivars for resistance. The differential set comprises a series of spinach cultivars (hybrids) that have different resistance patterns to the currently identified pathogenic races.
Currently there are 14 officially recognised races of Peronospora farinosa f. sp. spinaciae, designated races Pfs 1 to Pfs14 (Irish et al. Phtypathol. Vol. 98 pg. 894-900, 2008; Plantum NL (Dutch association for breeding, tissue culture, production and trade of seed and young plants) press release, “Benoeming van Pfs: 14, een nieuwe fysio van valse meeldauw in spinazie”, Sep. 19, 2012; Report Jim Correl (Univ. Arkansas) and Steven Koike (UC Cooperative Extension, Monterey County), “Race Pfs: 14—Another new race of the spinach downy mildew pathogen”, Sep. 18, 2012). Races 4 to 13 were identified between 1990 and 2011, while only recently another new Peronospora isolate has been identified, termed UA4410, which subsequently has been officially named Pfs14 by the International Working Group on Peronospora (IWGP) (Plantum NL (Dutch association for breeding, tissue culture, production and trade of seed and young plants) press release, “Benoeming van Pfs: 14, een nieuwe fysio van valse meeldauw in spinazie”, Sep. 19, 2012; Report Jim Correl (Univ. Arkansas) and Steven Koike (UC Cooperative Extension, Monterey County), “Race Pfs: 14—Another new race of the spinach downy mildew pathogen”, Sep. 18, 2012). All 14 officially recognized Pfs races are publicly available from the Department of Plant Pathology, University of Arkansas, Fayetteville, Ark. 72701, USA, and also from NAK Tuinbouw, Sotaweg 22, 2371 GD Roelofarendsveen, the Netherlands.
Newly identified Peronospora races can break the resistance of many spinach varieties that are currently used commercially worldwide, and they thus pose a serious threat to the productivity of the spinach industry. For this reason new resistance genes are very valuable assets. In order to confer a resistance that is as broad as possible, i.e. that confers resistance to as many Pfs races as possible, preferable to all known Pfs races, it is very useful to be able to stack different resistance genes against Peronospora infection in spinach. This is achieved by stacking various resistance genes which have overlapping resistance patterns. In this way it becomes more difficult for the pathogen to overcome the resistance.
Other important destructive foliar diseases in spinach include leaf spot diseases like Cladosporium leaf spot and anthracnose. Although a complex of fungi is associated with the Cladosporium leaf spot disease on spinach, Cladosporium variabile is believed to be the primary pathogen involved. Cladosporium belongs to a different class of fungi than the spinach downy mildew fungus, and fungicides effective against mildew are not necessarily effective against Cladosporium leaf spot. Early symptoms of infection occur as numerous small, circular white to yellow spots, beginning on the older leaves and progressing to the younger ones as the plant grows during the season. Spots often join together and become irregular in shape. When the fungus produces spores, the leaf spots become olive black in color from the spore masses. Generally, older leaves are killed, but in the case of severe infection all of the leaves on the plant may be affected and perish. Spinach anthracnose is caused by the fungus Colletotrichum dematium f. sp. spinaciae. Early symptoms of anthranose are small lesions and necrotic areas on leaves. These lesions advance to form large necrotic areas in the leaves in which characteristic structures of the fungus may be observed using a 10-20× hand lens or a stereoscope. Spores are produced abundantly during wet conditions. They are spread by wind and splashing water. Anthracnose of spinach is difficult to control. The increased use of wide densely planted beds and overhead irrigation creates an ideal environment for disease development. Tolerant varieties are not available, and chemical treatments are not effective. Cladosporium leaf spot and anthracnose often occur together on the same plants. Both diseases are responsible for significant economic losses in spinach production areas. Obtaining spinach varieties with a level of genetic resistance to Cladosporium and anthracnose is thus highly desired.
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.